kopper: Only include XCB headers on supported platforms

Fixes: 054bd2d3 ("kopper: copy a bunch of code for texture_from_pixmap")

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <!16675>
152 jobs for !16675 with fix-build in 39 minutes and 19 seconds (queued for 14 seconds)
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